Attempt to de-flake dart interop build

templates/tools/dockerfile/interoptest/grpc_interop_dart/build_interop.sh.template

@@ -25,4 +25,5 @@
   cp -r /var/local/jenkins/service_account $HOME || true
 
   cd /var/local/git/grpc-dart/interop
-  /usr/lib/dart/bin/pub get --verbose
+  # De-flake attempt: run the cmd one more time in case of transient failure
+  /usr/lib/dart/bin/pub get --verbose || /usr/lib/dart/bin/pub get --verbose

tools/dockerfile/interoptest/grpc_interop_dart/build_interop.sh

@@ -23,4 +23,5 @@ git clone /var/local/jenkins/grpc-dart /var/local/git/grpc-dart
 cp -r /var/local/jenkins/service_account $HOME || true
 
 cd /var/local/git/grpc-dart/interop
-/usr/lib/dart/bin/pub get --verbose
+# De-flake attempt: run the cmd one more time in case of transient failure
+/usr/lib/dart/bin/pub get --verbose || /usr/lib/dart/bin/pub get --verbose